If you're going for that length of time and collecting/returning from the same location, then book the car via

www.billiger-mietwagen.de

This is a broker who compares all the hire car companies and manages to elicit very competititive rates. The full insurance packages they offer are also very reasonable, so you can double up on your coverage if using a cc that also offers CDW as part of its conditions.

And then, as always, inspect the car meticulously upon collection and report any non-noted damage to staff.